This invention relates to a centrifugal actuator assembly and more particularly to a centrifugal actuator assembly for actuating a starting switch on an AC electric motor in response to the operating speed of the motor.
In many conventional electric motors, such as single phase induction motors, a rotating magnetic field is produced by means of a main winding and a starting winding included within the stator assembly of the motor. These windings are so designed and arranged within the stator that upon start up of the motor, sufficient starting torque is attained. Once the motor reaches a predetermined operational speed, a switch (referred to as a starting switch) is opened thereby to deenergize the starting winding by means of some device responsive to the rotational speed of the motor. Upon the motor slowing below another predetermined speed of operation (e.g., upon stopping), the starting switch is closed so that upon the motor being again energized the starting winding will also be energized so as to generate sufficient torque for starting purposes. The starting switch conventionally is operated by a centrifugal actuator rotatable with the rotor shaft of the motor, the centrifugal actuator being responsive to the speed at which the motor (i.e., the rotor shaft of the motor) is rotated.
As shown in U.S. Pat. No. 3,271,602 and 3,609,421, prior art centrifugal actuators were rotatable with a rotor shaft and included a centrifugal weight assembly. The latter included a pair of centrifugal levers pivotally mounted on the shaft and were rotatable therewith with the levers carrying a weight on their outer or free ends. Upon the shaft rotating at a predetermined speed, the weights are thrown outwardly by centrifugal force thus causing the levers to pivot which in turn effects axial shifting movement of a portion of the centrifugal actuator with respect to the rotor shaft. This axial movement of a portion of the centrifugal actuator assembly in one direction from a so-called starting position to a run position upon acceleration of the motor to its predetermined operational speed effects the opening of the starting switch through the operation of an appropriate linkage or the like. Typically, the centrifugal levers are resiliently biased inwardly toward the shaft and toward their starting position by means of extension springs. As the centrifugal actuator assembly is rotated to its desired predetermined operational speed, the centrifugal levers must overcome the bias of the extension springs to move outwardly from their starting to their run positions. Upon slowing of the motor, the extension springs bias the levers inwardly and effect axial movement of a portion of a centrifugal actuator assembly from its run position to its starting position which in turn closes the starting switch.
Typically, the extension springs used to bias the centrifugal levers inwardly toward their starting positions are conventionally wound wire extension springs having hook-type ends for connection between the opposed centrifugal levers and for transferring the load of the spring to the centrifugal levers. However, these hook ends on the extension springs introduce a relatively high stress concentration factors into the springs and, on occasion, have lead to the premature failure (breakage) of the spring. A broken spring in a centrifugal actuator assembly would, in some instances, prevent the motor from starting either due to premature deenergization of the starting winding or due to failure of the centrifugal actuator to close the starting switch upon shut down of the motor. Also, the centrifugal actuator levers are normally restrained on the actuator by the springs and in the event of a spring failure, the levers oftentimes are flung outwardly and are thrown into the stator windings shorting them and causing failure of the motor. Still further, pieces of the broken spring could work their way into the windings of the motor and cause an electrical failure of the motor.
Also, extension springs resulted in a non-linear application of biasing force on the centrifugal actuators. Typically, the coils of an extension spring are closely wound in their relaxed or unloaded position so that an initial load, referred to as initial tension, must be applied to the spring to separate its closely packed turns. This initial tension is usually specified to be present in extension springs so that the spring will accurately hold its free length. However, once the initial tension of the spring has been overcome, the spring will then linearly extend in such manner as is indicated by the spring constant for the spring. In effect then, extension springs have two different spring constants, one which is in effect until the initial tension of the spring has been overcome and the other of which is in effect during the remainder of the extension of the spring. Also, in certain conditions, extension springs may be overstressed thus permanently deforming them and permanently altering the force applied to the centrifugal actuator by the springs which in turn alters the centrifugal force (and the rotational speed of the rotor shaft) required to open the starting switch.